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Cobra 2000 No modulation
- Thread starter Brian G
- Start date
No modulation can be a variety of things. First thing to try is usually the easiest, swap the mike. If you don't have a spare that does raise the degree of difficulty a touch.
If you know the mike is good, start following the audio signal through the radio. Is it good where the "Dynamike" (mike gain) control connects to the main board? If yes, move to the next component in the chain and check if it's good leaving that. If no, you know the problem is somewhere between the Dynamike control and the mic jack. Or the control is just turned all the way down.
Now, if I misunderstood and you were looking for a component to replace as a "typical" failure, no clue.
If you know the mike is good, start following the audio signal through the radio. Is it good where the "Dynamike" (mike gain) control connects to the main board? If yes, move to the next component in the chain and check if it's good leaving that. If no, you know the problem is somewhere between the Dynamike control and the mic jack. Or the control is just turned all the way down.
Now, if I misunderstood and you were looking for a component to replace as a "typical" failure, no clue.
Try another mike. Mikes tend to break more frequently than radios.
Alternately, you could try this mike on a radio that you know is working.
Starting with the simplest-possible solution can save time and aggravation.
73
Alternately, you could try this mike on a radio that you know is working.
Starting with the simplest-possible solution can save time and aggravation.
73
Just trying to follow along.....
You say that receive audio works fine....
But PA does not work?
That would seem to indicate that the problem is getting the mic audio in to the audio amp.... the audio amp itself is working fine.
on the assumption that you have tried other working mics with no joy....
If it were mine, I'd be tracing pin 1 of the mic jack through to the volume pot then on to C108 and on towards TR23 & TR22 and making sure that TR24 is not pulling the mic audio down for some reason. Also make sure TR25 is not pulling the amplified mic audio down before it heads down toward the record circuit and the volume control.
Not that this is going to help.... just trying to learn to think my way through this... and hoping that might help you.
You say that receive audio works fine....
But PA does not work?
That would seem to indicate that the problem is getting the mic audio in to the audio amp.... the audio amp itself is working fine.
on the assumption that you have tried other working mics with no joy....
If it were mine, I'd be tracing pin 1 of the mic jack through to the volume pot then on to C108 and on towards TR23 & TR22 and making sure that TR24 is not pulling the mic audio down for some reason. Also make sure TR25 is not pulling the amplified mic audio down before it heads down toward the record circuit and the volume control.
Not that this is going to help.... just trying to learn to think my way through this... and hoping that might help you.
You are correct, RX is fine and radio dead keys normally, just no modulation on AM or SSB or PA. I am looking in that area between the mic jack and the audio chip. So you think IC6 is fine since the receive is ok? I did check the voltages across IC6 and they seem to be ok. I really think the issue is somewhere between mic jack and TR22. TR24 was originally clipped out of this radio but I put it back awhile ago and like I said I had no issues with it for a few years. And I did a complete electrolytic recap on it a few years back too.
Once I'm sure the mike is okay, checking the positive side of C18 would be next. Should have around 6 Volts DC or so. If it gets turned the wrong way, or just suffers infant mortality, this one shuts down mike audio when it shorts.
73
73
Gotta make a video showing how to use the "Manual Digital Analog Signal Injector".
It consists of a thin screwdriver blade with one finger laying alongside it. Touching it to first the output, then the input of each mike amplifier stage can serve to narrow down the point of failure.
On pin 4 of the TA7222 audio chip you'll get a loud hum and usually a squeal along with it, both in receive mode and AM transmit, but not sideband transmit.
On the center pin of TR24 you should get a loud hum on your transmit carrier so long as the mike gain is turned up.
If you get sound at TR24 the problem is between there and the mike socket. There's no point to molesting the TA7222 audio chip if you have normal receiver audio. Just an example of two points of interest in the larger picture.
73
It consists of a thin screwdriver blade with one finger laying alongside it. Touching it to first the output, then the input of each mike amplifier stage can serve to narrow down the point of failure.
On pin 4 of the TA7222 audio chip you'll get a loud hum and usually a squeal along with it, both in receive mode and AM transmit, but not sideband transmit.
On the center pin of TR24 you should get a loud hum on your transmit carrier so long as the mike gain is turned up.
If you get sound at TR24 the problem is between there and the mike socket. There's no point to molesting the TA7222 audio chip if you have normal receiver audio. Just an example of two points of interest in the larger picture.
73
Well, if the previous owner did any rework, it should stand out - fresh solder or extra flux that looks "newer" than the other places.
It's my thinking that, if you have receive(r), just no modulation - everything from the PA jack onto the AM Regulator pairing is suspect.
Can mean they had a "Talkback" feature and when they removed it back to stock, they resoldered that wire that would normally go to the board from the CB/PA switch - to the wrong solder pad.
Also, CB/PA switch is a TWO pole unit, so the TX transistor and Pin 5 to one pole, while Audio Out goes to PA Speaker and Inside speaker (Internal) - but uses Pin 3 for it's "Return" (Thru Pin 4 back into Ground in the radio)
When you TX? The "ground" opens, and Pin 5 is active as the TX switch.
Make sure you aren't sending Speaker ground thru the wrong wire - or if they did a 5-wire to 4 wire downsize, you have to "Break out and break apart" the wiring routing to reset back to OEM.
It's my thinking that, if you have receive(r), just no modulation - everything from the PA jack onto the AM Regulator pairing is suspect.
Can mean they had a "Talkback" feature and when they removed it back to stock, they resoldered that wire that would normally go to the board from the CB/PA switch - to the wrong solder pad.
Also, CB/PA switch is a TWO pole unit, so the TX transistor and Pin 5 to one pole, while Audio Out goes to PA Speaker and Inside speaker (Internal) - but uses Pin 3 for it's "Return" (Thru Pin 4 back into Ground in the radio)
Make sure you aren't sending Speaker ground thru the wrong wire - or if they did a 5-wire to 4 wire downsize, you have to "Break out and break apart" the wiring routing to reset back to OEM.
Other options?
Tr41 and TR42, - else very little gets in the way.
Hows' your 8V TX - get a Red light?
A catastrophic failure of say a diode - would affect AM - but not SSB - UNLESS TR41 blew open - means no signal or any voltage gets to the Driver and Final.
IF R193 is still in there, that would allow you to "hear" SSB signal very weakly on another close-by radio if you threw a dummy load on the back at the antenna jack of THIS radio.
So no signal on SSB usually indicates failed MODE, (blown trace/Burnt switch contact) or TR41 and TR42 are suspect. Carrier but no Modulation - check Mic Amp and Mic routing.
So logic to me, says TR41, TR42 and MODE switch need to be checked out.
Mode switch first, for if SSB doesn't work, the switch is suspect, the SSB signal arrives to the TX MIXER - so the entire strip amplifies signal. - also failed TX power to this strip also affects that "modulation"
You say you get Carrier, just no mudulation - so that rules much of anything else except MODE, TR41 or TR42.
Did you put an external speaker on the PA jack and try to see if the CB/PA switch is working?
IF you get PA, then most of the Mic strip is functioning. (I said MOST)
In SSB mode, no modulation usually means Failed Mic Amp strip. But if it was working then just "quit" I'd look into making sure you're in LOCK (Red Light), and/or the MODE switch didn't fail - blow open - to stop sending power to the Driver and Final - which in themselves can - if they fail - will take out the switch too - when they blow shorted. You get Carrier so it's does not appear as a Driver or Final problem. - But you should check the "center leg" and make sure the Driver and Final are getting power. MODE switch in SSB.
IN the above Mic Amp strip - TR22 and TR21 work together - AND - if any part has failed shorted - they tend to OVERAMPLIFY - as in you can see they have bias but if higher by a volt of so, they will "CLIP" and the audio disappears - simply because the amps are working now like switches than LINEAR signal amps.so you don't hear ANYTHING. You said a Recap a few years ago, so otherwise the Mic amp if the PA amp works, is still Ground Zero amongst the others...
This effect can be reciprocal - one failed mic amp resistor - the train stops dead in it's tracks...
IT's why I asked about the "RED TX light" if the RED light is on, you're in LOCK so the MB3756 would let you get AM - you say you get carrier - so I can presume you have LOCK and the radio wants to transmit.
In SSB - if the switch went - blown open - you'd wouldn't get any power back there in the SSB modes (Both USB and LSB) so no SSB.
But AM? You might because of the poles - the wires from the switch route thru so if one wire opened the pole could still send power thru the other mode (in this case AM)
If you can put a Speaker on the PA switch and it works and the MIC amp seems to be ok - well, then it narrows this down to TR42 being shorted or TR41 - since you have AM TR41 works - to a degree - and that requires TR42 - but if TR42 is shorted - doesn't matter TR41 is still on, tracking what Bias is being sent thru TR42.
Tr41 and TR42, - else very little gets in the way.
Hows' your 8V TX - get a Red light?
A catastrophic failure of say a diode - would affect AM - but not SSB - UNLESS TR41 blew open - means no signal or any voltage gets to the Driver and Final.
IF R193 is still in there, that would allow you to "hear" SSB signal very weakly on another close-by radio if you threw a dummy load on the back at the antenna jack of THIS radio.
So no signal on SSB usually indicates failed MODE, (blown trace/Burnt switch contact) or TR41 and TR42 are suspect. Carrier but no Modulation - check Mic Amp and Mic routing.
So logic to me, says TR41, TR42 and MODE switch need to be checked out.
Mode switch first, for if SSB doesn't work, the switch is suspect, the SSB signal arrives to the TX MIXER - so the entire strip amplifies signal. - also failed TX power to this strip also affects that "modulation"
You say you get Carrier, just no mudulation - so that rules much of anything else except MODE, TR41 or TR42.
Did you put an external speaker on the PA jack and try to see if the CB/PA switch is working?
IF you get PA, then most of the Mic strip is functioning. (I said MOST)
In SSB mode, no modulation usually means Failed Mic Amp strip. But if it was working then just "quit" I'd look into making sure you're in LOCK (Red Light), and/or the MODE switch didn't fail - blow open - to stop sending power to the Driver and Final - which in themselves can - if they fail - will take out the switch too - when they blow shorted. You get Carrier so it's does not appear as a Driver or Final problem. - But you should check the "center leg" and make sure the Driver and Final are getting power. MODE switch in SSB.
IN the above Mic Amp strip - TR22 and TR21 work together - AND - if any part has failed shorted - they tend to OVERAMPLIFY - as in you can see they have bias but if higher by a volt of so, they will "CLIP" and the audio disappears - simply because the amps are working now like switches than LINEAR signal amps.so you don't hear ANYTHING. You said a Recap a few years ago, so otherwise the Mic amp if the PA amp works, is still Ground Zero amongst the others...
This effect can be reciprocal - one failed mic amp resistor - the train stops dead in it's tracks...
IT's why I asked about the "RED TX light" if the RED light is on, you're in LOCK so the MB3756 would let you get AM - you say you get carrier - so I can presume you have LOCK and the radio wants to transmit.
In SSB - if the switch went - blown open - you'd wouldn't get any power back there in the SSB modes (Both USB and LSB) so no SSB.
But AM? You might because of the poles - the wires from the switch route thru so if one wire opened the pole could still send power thru the other mode (in this case AM)
If you can put a Speaker on the PA switch and it works and the MIC amp seems to be ok - well, then it narrows this down to TR42 being shorted or TR41 - since you have AM TR41 works - to a degree - and that requires TR42 - but if TR42 is shorted - doesn't matter TR41 is still on, tracking what Bias is being sent thru TR42.
I am not getting anything even through the PA switch and an external speaker in the PA jack. It is receiving normally on AM and SSB and the AM carrier is normal--just no modulation at all. And nothing on SSB either.
So the CB/PA switch, you checked, seems to work ok.
I wonder if you even hear a "thunk" from the speaker?
Then look around that area for fresh work as say a diode or wire, here's the thought.
IF they put in a variable key, you'd see work around the Final and Driver section.
You have Transmit, you hear yourself in another radio (correct?) (if so) that means you have Audio going thru there - into the Pass transistors.
You checked and you HAVE NO audio in the PA side, which tells me several things...
Possible...
... they DID do some type of talkback mod to hear themselves - only it's failed. OR they removed it. And did not put it back correctly...
Or you have a bad audio chip - The biggest clue is you don't hear the "thunk" from the speaker when you power up.
Onto... bad soldering - broken wire in the audio amp area. Open circuit.
Just three choices, there are output caps in that area too, look above in the schematic snips I've posted.
So if the Caps --- have they been replaced?
How about that "thunk"? from the speaker when you turn it on.
Does the Audio Chip get hot, or warm?
How about if it's stone cold, Cold - dead it's toast - replace the 7222 Flat-pak with another one. and don't just turn on the power right away, review the traces around it and question any spots - you can power up the radio without the amp in place and observe power flows and voltages to the pins on the side edge - make sure they are not all the same voltage , and you should have power only to one, with little to none on the others.
Hot? Look for bad soldering or panel to part, missing/displaced thermal mica tab - possibly shorting out to side panel - you can check for this by testing for "ground" on tab, as the same value on the side metal panel - if the radio is stock should be 4K to as much as 15K ohmic reading across there.- if it's a dead short, possibly took out the chip - so it will have to be investigated and fixed as needed.
Warm? It' may be in idle - look for the routing of the pins and for open blown traces from it, any surge can pop the trace like a fuse - and even damage the chip so it may "boot" but have no amp inside it. So warm doesn't guarantee of anything just that the amp is biased to work at idle.
IF you don't remember hearing one of the above, and it feels like something isn't right - it usually is...then it may be overdue for some caps and or a chip needs to be checked and replaced.
.
I wonder if you even hear a "thunk" from the speaker?
Then look around that area for fresh work as say a diode or wire, here's the thought.
IF they put in a variable key, you'd see work around the Final and Driver section.
You have Transmit, you hear yourself in another radio (correct?) (if so) that means you have Audio going thru there - into the Pass transistors.
You checked and you HAVE NO audio in the PA side, which tells me several things...
Possible...
... they DID do some type of talkback mod to hear themselves - only it's failed. OR they removed it. And did not put it back correctly...
Or you have a bad audio chip - The biggest clue is you don't hear the "thunk" from the speaker when you power up.
Onto... bad soldering - broken wire in the audio amp area. Open circuit.
Just three choices, there are output caps in that area too, look above in the schematic snips I've posted.
So if the Caps --- have they been replaced?
How about that "thunk"? from the speaker when you turn it on.
Does the Audio Chip get hot, or warm?
How about if it's stone cold, Cold - dead it's toast - replace the 7222 Flat-pak with another one. and don't just turn on the power right away, review the traces around it and question any spots - you can power up the radio without the amp in place and observe power flows and voltages to the pins on the side edge - make sure they are not all the same voltage , and you should have power only to one, with little to none on the others.
Hot? Look for bad soldering or panel to part, missing/displaced thermal mica tab - possibly shorting out to side panel - you can check for this by testing for "ground" on tab, as the same value on the side metal panel - if the radio is stock should be 4K to as much as 15K ohmic reading across there.- if it's a dead short, possibly took out the chip - so it will have to be investigated and fixed as needed.
Warm? It' may be in idle - look for the routing of the pins and for open blown traces from it, any surge can pop the trace like a fuse - and even damage the chip so it may "boot" but have no amp inside it. So warm doesn't guarantee of anything just that the amp is biased to work at idle.
IF you don't remember hearing one of the above, and it feels like something isn't right - it usually is...then it may be overdue for some caps and or a chip needs to be checked and replaced.
.